This invention relates generally to shape memory materials, and more particularly relates to ceramic shape memory materials.
Ceramic shape memory materials, or shape memory ceramics (SMC) are ceramic compounds that can change their shape by application of a load to the ceramic material, and that can subsequently return to their original shape by either heating or unloading of the ceramic material. The so-called shape memory effect refers to a cycle of load application and heating for cyclic shape change, while the characteristic of superelasticity refers to the cycle of loading and unloading for cyclic shape change. The physical mechanism underlying both of these cyclic Shape changes is related to a thermoelastic martensitic transformation between two crystallographic phases. The most widely employed shape memory materials are metals, and in particular metal alloys. Shape memory alloys (SMAs) are well-known for their ability to transform between the martensitic and austenitic crystallographic phases. But conventional SMA structures are characterized by relatively low transformation stresses and correspondingly low energy dissipation capabilities. In contrast, some ceramic shape memory materials have been shown to be capable of exhibiting reversible martensitic transformation with high stresses, offering the prospect of improved energy dissipation over that of conventional SMAs and the ability to particularly address applications in, e.g., actuation, energy harvesting, and mechanical energy damping.
For example, the family of zirconia-based shape memory ceramics has been well-studied for applications in sensing, actuation, and mechanical energy damping. Zirconia exhibits a reversible thermoelastic transformation between a high-temperature tetragonal phase, referred to as austenite, and a low-temperature monoclinic phase, referred to as martensite, with associated shear strains of up to 15%. This strain can be fully recovered by release of an applied stress that induced the strain, to cause a reverse martensitic transformation, as a result of the superelastic capability of zirconia.
It is found that for many shape memory ceramics like zirconia, the martensitic transformation and its associated shape change generally leads to substantial internal stresses. As a result, many shape memory ceramics, which are in general brittle materials, have a tendency to fracture or crack during martensitic transformation. For example, at strains of only about 1%-2%, cracking occurs in polycrystalline zirconia after only a few transformation cycles. This cracking originates from the large mismatch stress that accumulates in neighboring crystalline grains, which have differing crystal orientations, during a martensitic transformation. Thus, although ceramic materials can in principle exhibit superior shape memory and superelastic properties with useful transformation shape recovery, such was not in general historically achievable due to the inherent brittle nature of such ceramic materials.
It has been proposed to mitigate the condition of shape memory ceramic material cracking by employing small-volume ceramic structures that include only a few crystalline grains within. Such a ceramic structure has a large free surface area and little grain boundaries. This enables relaxation of transformation stress in the structure and hence minimization of the formation of inter-granular fractures during martensitic transformation, leading to a robust shape memory ceramic structure capable of many superelastic cycles with large strains. The combination of high strength, large recoverable strain, large energy damping, and light weight that such structures exhibit are interesting for many challenging applications, particularly in energy damping and absorption.
But such ceramic structures generally are not in a geometric form that can be employed in many engineering applications, particularly large-scale engineering applications. Further, the production of such micro-scale ceramic structures is not in general amenable to large-volume production methods. As a result, the challenge of successful integration of shape memory ceramic materials into many energy-damping systems remains unmet.